Media captionBarack Obama described his "vision of a world without nuclear weapons"

US President Barack Obama says he is pushing for "a world without nuclear weapons", making direct appeals to North Korea and Iran.

He also pledged to work with Russia and China, speaking ahead of a summit in Seoul aimed at reducing the risk of nuclear terrorism.

He emphasised the US's unique position to seek change but said "serious sustained global effort" was needed.

The meeting is being attended by representatives from some 50 countries.

Speaking to students at Hankuk University, Mr Obama reiterated the commitment of the US as ''the only nation to have ever used nuclear weapons'' to reducing its nuclear arms stockpile.

He also spoke, he said, as a father who did not want to see his daughters growing up in a world with nuclear threats, a comment which drew applause from his student audience.

Analysis

By Jonathan MarcusBBC diplomatic correspondent

Ever since the collapse of the Soviet Union in 1991, there have been fears about nuclear materials falling into the hands of terrorists and extremists.

In the United States, such concerns took on added significance in the wake of the 9/11 attacks on New York and Washington. Washington and its key allies - like the UK - are convinced that al-Qaeda is seeking to obtain material for a nuclear or radiological bomb.

In April 2010, President Barack Obama convened a summit in Washington that set the ambitious goal of securing all vulnerable nuclear materials worldwide within four years. Some progress has been made - for example Chile returned highly enriched uranium to the US; Kazakhstan has moved spent fuel to a secure depot; and Ukraine has transferred fissile material to a Russian storage site.

But progress has been patchy, in part because the initial goal set in 2010 was vague without a detailed timeline or work plan. Now, two years on, this Seoul summit will try to create new momentum.

The US president said he was looking forward to meeting newly-elected Russian President Vladimir Putin in May to discuss further nuclear arms cuts.

Mr Obama would seek to follow on from the New Start (Strategic Arms Reduction Treaty) pact he struck in 2010 with outgoing Russian leader Dmitry Medvedev in 2010, he said.

The New Start deal agreed between Washington and Moscow was intended to replace its lapsed predecessor, Start.

It trims US and Russian nuclear arsenals to 1,550 deployed nuclear warheads - a cut of about 30% from a limit set previously.

The treaty would also allow each side visually to inspect the other's nuclear capability, with the aim of verifying how many warheads each missile carries.

In addition, there will be legally binding limits on the number of warheads and missiles that can be deployed on land, on submarines, and on bombers, at any one time.

Warning to Pyongyang

In Asia, President Obama said, the US has invited China to work with Washington and ''that offer remains open''.

"We both have an interest in making sure that international norms surrounding non-proliferation, preventing destabilising nuclear weapons, is very important," he said ahead of a meeting with Chinese President Hu Jintao.

Today, we say: Pyongyang, have the courage to pursue peaceBarack Obama, US President

He also addressed North Korea's nuclear ambitions directly in his speech at Hankuk University, saying that the US has ''no hostile intent'' towards the country, but ''there will be no rewards for provocation''.

He warned Pyongyang that its planned long-range missile launch would only increase its isolation.

Pyongyang says it is preparing to launch a long-range missile which it says will put a satellite in orbit.

''You can continue with the road you are on but we know where that leads,'' he said, addressing the North Korean leaders directly.

''Today, we say: Pyongyang, have the courage to pursue peace.''

Earlier, he and South Korean President Lee Myung-bak said North Korea risked further sanctions and isolation if it did not cancel its launch plans.

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Media captionPresident Obama: ''Today, we say: Pyongyang, have the courage to pursue peace"

The launch will contravene an agreement Pyongyang reached last month which would have seen it receive food aid in exchange for a partial freeze on nuclear activities and an end to ballistics tests.

The North also agreed to allow UN inspectors in, the US said.

The invitation comes three months after Kim Jong-un came to power following the death of his father, Kim Jong-il.

The North said the launch - between 12 and 16 April - would mark the 100th birthday of former leader Kim Il-sung.

South Korea has warned it will shoot down the rocket if it strays over its territory.

"We are preparing measures to track the missile's trajectory and shoot it down if it, by any chance, deviates from the planned route and falls into our territory," a defence ministry spokesman said.

The launch site is in north-western North Korea, not far from the Chinese border.

'Time is short'

Addressing Iran, Mr Obama said there was still time to resolve the impasse over its nuclear programme through diplomacy.

"But time is short,'' he warned. ''Iran must act with the seriousness and sense of urgency that this moment demands."

Iran insists there is no military element to its programme but Western powers fear it is constructing nuclear weapons.

"Today, I'll meet with the leaders of Russia and China as we work to achieve a resolution in which Iran fulfils its obligations," Mr Obama added.

Despite lofty announcements it may prove difficult to achieve significant progress at the summit, says the BBC's Jonathan Marcus.

The summit agenda is to be expanded to include a wide variety of radiological materials which terrorists could use to make a dirty bomb - one that spreads radiological contamination rather than initiating a nuclear explosion

But experts say there is unlikely to be agreement on converting all nuclear power stations to use low-enriched fuel.

Nor will there be agreement on common standards for nuclear security.

Some countries see this whole process as highly intrusive.

And there is still no common appreciation of the level of threat posed by nuclear terrorism, our correspondent says.